Modelling of convection, diffusion and binding of chlorides in concrete during wetting-drying cycles

Abstract

Moisture convection in concrete is an important factor affecting the chloride diffusion in concrete. This paper presents a comprehensive study on the transport of moisture, carbon dioxide and chloride ions in concrete when subjected to wetting and drying cycles. A numerical model considering not only the transport of individual components but also the interaction between them is developed. The latter includes the concrete carbonation, chloride binding, the effect of concrete carbonation on chloride binding capacity, and the time-varying boundary conditions of carbon dioxide and chloride caused due to wetting and/or drying. The results obtained from the study demonstrate that when moisture convection and concrete carbonation in the convection zone near the exposed surface are involved the chloride indeed exhibits an increase in a thin layer next to exposed surface before it decreases with the distance as observed in many experiments. • A comprehensive model describing transport of moisture, carbon dioxide and chloride is developed. • Moisture convection is found to have great impact on the transport of carbon dioxide and chlorides. • It demonstrates that the peak point of chloride profile indeed occurs at inner concrete near the surface. • Chloride concentration at peak point increases with increased number of wet-dry cycles. • It appears necessary to split bound chloride into physical and chemical bound chlorides.